Re: Avengers Score : Singular, Or Multiple Composer?
Giacchino is really a top choice. His works have been identifiable (as being composed by him), and yet separate and distinct in their sound. His themes mix a degree of epicness with drama and heart (Star Trek displayed this quite excellently), and action themes aren't at all 'out of his comfort zone' (MI3). Mystery and character themes have been explored with LOST, and Up was just a bundle of joy.
Considering Whedon's on the project - I don't recall a 'constant' composer in his past projects.
I think two collaborative composers would work well for the film, though.